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DNA Spells Evolution

JANET BROWNE: If Darwin came back today, what would he be most excited by in modern science? I think it would be the whole world of genetics. That’s the answer to all the big questions he had—how does variation emerge? And how could that be transmitted?

NARRATOR:
Let’s return to our brown bears stranded in the Arctic to consider the impact of genetics on our understanding of evolution. Each bear is made up of cells. And if we take a brown bear cell, and tunnel into its nucleus, we find DNA—the molecule with the genetic instructions for building, in this case, a brown bear, written in a four-letter code.

Now, the thing about DNA—it’s not perfect. When it’s copied, mistakes get made. Mutations, in other words, that sometimes affect an organism’s traits and that sometimes can be passed from parent to offspring. So the variation at the heart of evolution – it’s genetic variation. Slight differences in DNA that—for example—could make some bears a bit lighter in color, a bit more insulated against the cold, and a bit more capable of digesting fattier foods like seals.

DANIEL MATUTE: Evolution is essentially any change in the genetic composition of a population.

NARRATOR:
Mutations are random, so they’re not always helpful. But the bears with mutations that gave them some advantage for Arctic living survived and reproduced more often than bears without them. They passed the genes responsible for those adaptations onto their cubs. Over generations, more bears inherited and elaborated on these and other changes in the DNA. The eventual result—a polar bear. And when we tunnel into its cells, we find polar bear DNA.

These days, most scientists use DNA sequences to build phylogenetic trees of living organisms. Their analyses get complicated fast because DNA can mutate rapidly and unpredictably throughout the genome. So we’ve simplified things for our puzzles. Use the magnifying glass to compare the actual DNA of different species. We’ve selected small fragments from much longer sequences for you to analyze.

The numbers beneath the DNA sequences correspond to the position of each nucleotide. Instead of using physical traits as we did in previous puzzles, we’re using nucleotides and their corresponding position numbers. Often, one of the species will be what’s called an outgroup—you can think of it as the reference to compare all the other species against. So it goes to one side of the tree. And here’s the basic rule:

SCOTT EDWARDS: The more closely related two species are – their DNA is also going to be very similar.

NARRATOR:
And the more time that’s passed from when two species split off from one another, the more time there’s been for mutations to accumulate, so the more different the DNA will be.

Think of DNA as the raw material that—across billions of years—evolution has molded and built into countless forms of life. We are all related, tied to one another by the most spectacular of double helical threads.